The present invention relates to a sapphire substrate having a heteroepitaxial growth surface, a semiconductor device having a group III nitride single-crystal layer grown on a substrate, an electronic component having a crystal layer of a nitride material on a crystal substrate having a rhombohedral crystal structure, and a crystal growing method.
Gallium-nitride-based materials have received a great deal of attention as semiconductor materials having large band-gap energy and high melting point. For example, a device is manufactured using an InGaAlN crystal. In manufacturing this device, a crystal film need be heteroepitaxially grown on a substrate. This is because there is no crystal substrate formed from a gallium-nitride-based crystal. Conventionally, a corundum (sapphire) substrate is mainly used as a substrate to heteroepitaxially grow a crystal film, and its (0001) plane (C plane) is used as a heteroepitaxial growth surface for growing a crystal of a nitride material, i.e., a heterogeneous material.
Normally, GaN, which most easily grows in this crystal system, is grown first on a sapphire substrate. This is because (1) to grow a mixed crystal, the compositional control is difficult, (2) a high temperature of 1,300xc2x0 C. or more is required to grow AlN, and (3) to realize a high N equilibrium vapor pressure required in the growth of InN, the supply ratio N/In of the nitrogen material to the In material must be as high as about 1,000,000, and this reduces the growth rate to 100 nm/h or less, resulting in an industrial disadvantage.
Even in growing GaN, when a single crystal is directly grown on a substrate, only a crystal with a rough surface is obtained. It is difficult to form a crystal on such a surface. Even when a crystal is formed, the thickness of the formed layer is nonuniform, which makes it impossible to control the thickness in growth.
In addition, when a single crystal is directly grown, a crystal boundary is generated. This forms defects to largely damage the optical/electrical characteristics of the crystal. For the above reasons, a directly grown crystal cannot be used to manufacture a device.
To solve these problems, a two-step growing method has been used. This two-step growing method will be briefly described. GaN or AlN, which is polycrystalline, amorphous, or a mixture of these states, is grown to a thickness of 20 to 50 nm on a substrate at a low temperature (low-temperature buffer layer growth). The crystal is then converted into a single crystal at a high temperature (annealing), and high-quality single-crystal GaN is grown on it (actual growth). In this case, threading dislocations are present at a density of 108 to 1010/cm2 in the film because of a lattice-mismatch ratio of 13.8% between the substrate and GaN. These dislocations degrade the electrical/optical characteristics of GaN.
To reduce the dislocation density, a method appropriately using selective growth and lateral growth has been proposed. That is, GaN with a thickness on the submicron order is grown by the above-described method. Next, a large number of stripe-shaped patterns of SiO2 are formed parallel on the GaN film as a selective growth mask. GaN is grown under such conditions that it readily grows in a direction (lateral direction) perpendicular to the film thickness direction between the stripes. As a result, GaN films grown laterally from both sides of each stripe-shaped SiO2 film merge, so the entire surface of the substrate is covered with GaN.
To obtain a high-quality crystal, the GaN film must be as thick as 100 to 200 xcexcm. However, the dislocations in the underlying GaN film remain in the crystals on the window portion. In addition, the merge portion of the GaN films contains numerous defects because the lattice constant of the substrate does not match that of the heteroepitaxially grown GaN film. A high-quality crystal is present only between the merge portion and the SiO2 window portion. Even this portion has dislocations at a density of about 104 to 106/cm2. Furthermore, the high-quality crystal portion has only a 5-xcexcm wide stripe shape. This width cannot be increased because of limitations on the lateral growth technique.
A semiconductor laser structure is formed on the substrate with the GaN film deposited by the above-described method. A DVD apparatus which is regarded as the most important application purpose of the device requires a device lifetime of 100,000 hrs at an operation temperature of 50xc2x0 C. and optical output of 30 mW, though the actual device lifetime is several hundred hrs at maximum, i.e., shorter by two or more orders of magnitude. This may be caused by distortion in the high-quality crystal portion and spread of dislocations on both sides of the high-quality crystal portion. Hence, the above-described method is still insufficient in terms of crystallinity.
Additionally, since the above-described crystal growing method requires three steps: (1) two-step growth comprising low-temperature buffer growth, and annealing and actual growth, (2) selective growth film formation and pattern formation, and (3) lateral growth, the process is complicated and the growth cost becomes high.
As a lattice-match substrate, a (01{overscore (1)}0) plane (to be referred to as an M plane hereinafter) is present in a sapphire substrate. The present inventors have conventionally proposed this M plane as a heteroepitaxial growth surface (U.S. Pat. No. 5,006,908). At the time of this proposal (filed Feb. 13, 1989), not the two-step growth but direct growth had been executed to directly grow GaN on a substrate at a high temperature without intervening a GaN layer grown at a low temperature. In that age, a GaN crystal having satisfactory characteristics such as good surface planarity, low residual carrier concentration, high mobility, and high-intensity photoluminescence was obtained on the M plane rather than on the sapphire (0001) plane (to be referred to as a C plane hereinafter).
To further improve the crystallinity of GaN grown on the sapphire M plane, the two-step growth was executed using the sapphire M plane. The tilt angle of the substrate surface used at this time from the M plane was 0.3xc2x0 or less in each orientation. The substrate had a plane that could normally be called a (01{overscore (1)}0) just plane. However, the surface of the obtained GaN crystal was rougher than that of a GaN crystal obtained by two-step growth on a sapphire C plane.
This is because twin crystals are easily formed on a substrate having a sapphire (01{overscore (1)}0) just plane, as shown in FIG. 8. Twin crystals are formed because on the (01{overscore (1)}0) just plane substrate, the c-axis of GaN tilts at 32xc2x0 from the normal direction of the substrate surface to the [2{overscore (1)}{overscore (1)}0] and [{overscore (2)}110] directions of the sapphire substrate. That is, since the c-axis can be set in the two orientations, twin crystals are generated. Consequently, when a GaN buffer layer is formed on the substrate having the sapphire (01{overscore (1)}0) just plane, and a gallium nitride crystal is grown on the GaN buffer layer, crystals grow in two directions to form twin crystals, as shown in FIG. 8.
Generally, to improve the crystallinity in heteroepitaxial growth, an off substrate that tilts with respect to the just plane through an appropriate angle (called an off angle) is used. In this case, the off angle is not so large and normally about 1xc2x0 to 5xc2x0. When GaN was actually heteroepitaxially grown on a sapphire M substrate having such an off angle, no effect for suppressing twin crystal formation was obtained. This fact was revealed for the first time by actually growing the crystal.
As described above, conventionally, since it is difficult to obtain a nitride semiconductor with good crystallinity, a device having satisfactory characteristics can hardly be obtained using a nitride material such as a wide-gap semiconductor having a wide-band-gap energy.
The present invention has been made to solve the above problems, and has as its object to obtain a device having more excellent characteristics and having satisfactory electrical/optical characteristics and long lifetime, such as a nitride semiconductor device formed from, e.g., InGaAlN, by using a nitride material such as a nitride semiconductor.
In order to achieve the above object, according to the present invention, there is provided a sapphire substrate having a heteroepitaxial growth surface, wherein the heteroepitaxial growth surface is parallel to a plane obtained by rotating a (01{overscore (1)}0) plane of the sapphire substrate about a c-axis of the sapphire substrate through 8xc2x0 to 20xc2x0 in a crystal lattice of the sapphire substrate.